In one type of commonly used electrical motors, a stator comprises windings in which an electrical field creates a rotating magnetic field. Inside, or circumferentially outside the stator, a rotor is rotationally attached to rotate under influence of the magnetic field. Various principles exist. In a synchronous motor, the rotor is magnetised, or comprises a set of permanent magnets. This type of motor is simple and reliable and the rotational speed of the rotor corresponds to the rotational speed of the electrical field in the windings of the stator. In certain applications, however, the synchronous motor has an inappropriate start-up characteristic.
In asynchronous motors, the rotor comprises substantially longitudinally extending windings which in axially opposite ends of the rotor are interconnected by short circuit rings. Typically, a rotor for an asynchronous motor comprises a rotor core made from a magnetically conductive material and a squirrel cage wherein the windings and short circuit rings are moulded in one piece from an electrically conductive material, e.g. aluminium. The rotor could be laminated from sheets of a metal, wherein each sheet comprises an opening which, in combination with other sheets, form conductor slots extending axially throughout the rotor. After the assembly of the sheets into a rotor core, conductive bars constituting the windings are moulded directly into the conductor slots using the slots as a mould, and the short circuit rings are moulded in one piece with the bars. In use, an electrical current is induced into the windings of the rotor by the magnetic field generated in the stator, and due to a shift between the electrical field in the windings of the stator and in the windings of the rotor, the rotor starts to rotate. Such motors have good start-up characteristics but in order to continue the induction of an electrical field into the windings of the rotor, the electrical field of the stator must continuously move relative to the windings of the rotor. The rotational speed of the rotor will therefore always be lower than the rotational speed of the electrical field in the stator. To increase the speed of the rotor, a rotor for a line-start motor comprises, in addition to the windings, a set of permanent magnets, and a line-start motor thereby combines the advantages of synchronous and asynchronous motors.
In manufacturing of line-start motors, the fixation of permanent magnets in the rotor core is a sensitive process. Since an increased temperature during moulding of the squirrel cage may influence or even destroy the permanent magnets, it is desired to insert the magnets into the core after the moulding of the squirrel cage. Therefore, the core is typically made with slots for the magnets and with openings in an end face of the rotor. The openings are large enough to allow the magnets to be inserted into the slots after the moulding process. When the magnets are inserted into the slots, they must be solidly fixed to avoid displacement of the magnets during rotation of the rotor. For that purpose, some rotors comprise terminating end plates which close the openings or at least reduce the size of the openings to prevent the magnets from falling out of the slots. In the heretofore known motors, the end plate is joined e.g. by a traditional rivet or nail which extends through an opening in the endplate and down into the core of the rotor wherein the rivet is anchored, or the end plate is glued onto the end face. In any case, the attachment of the end plate to the end face of the rotor increases the complexity and costs of the manufacturing of the rotor.